1. Field of the Invention
The present invention relates generally to the production of dialkyl thionocarbamates, also sometimes referred to as thiourethanes. Compounds of this type are defined by the general formula ##EQU1## where R and R' are alkyl radicals. These products are known to be useful as flotation reagents in the recovery of minerals from ores and concentrates.
In its more specific aspects, the present invention relates to the production of high purity dialkyl thionocarbamates from an alkali metal xanthate and an amine in a one-stage, catalyzed reaction. It has been determined, for example, that isopropyl ethyl thionocarbamate made by the process of the present invention is a superior flotation reagent for certain ores, when compared to a commercial grade of the same reagent.
2. Prior Art
The production of dialkyl thionocarbamates, and their use as flotation reagents, is well known in the art. In essence, an alkali metal xanthate is first reacted with an alkyl halide to form a dialkyl xanthate. This is then reacted with an aliphatic amine to produce the desired compound and a mercaptan. The latter can be distilled off, and the organic and aqueous phases separated. In practice, however, problems arise.
In the initial formation of the alkali metal xanthate by reaction of an alkali metal hydroxide with carbon disulfide and an aliphatic alcohol, temperatures must be closely controlled to avoid significant trithiocarbonate formation. Close temperature control is also necessary in the intermediate reaction with an alkyl halide. The amine is then introduced and, after reaction, the organic reaction product is recovered. However, this is said to contain only 50-90% of the desired ester, the remainder being mainly alkyl dithiocarbamate, dialkyl xanthate and dialkyl trithiocarbonate.
In the prior art, the selection of reaction conditions always involved a trade-off between obtaining a reasonable reaction rate and yield of desired product. High temperatures improved reaction rate but also encouraged side reactions and decomposition of xanthate. The same is true of long reaction times. In one regard, the present invention is surprising in using both higher temperatures and longer times than would be taught by the conventional wisdom, while still producing a high purity product.
Prior workers have argued that such an impure ester is advantageous, in that costly and time consuming purification steps are avoided. It is said that no benefit is gained by isolating and purifying the intermediates, so that a pure ester could be obtained. Such purification steps would indeed be costly, but in accordance with the present invention, a high purity product is obtained and the only purification needed is washing with water.